FWKNOP

The main application of fwknop is to protect services such as SSH with an additional layer of security in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) much more difficult. It implements an authorization scheme known as Single Packet Authorization (SPA) that requires only a single encrypted packet to communicate various pieces of information including desired access through a Netfilter policy and/or specific commands to execute on the target system.